1. FIELD OF THE INVENTION
This invention relates in general to the construction of sound pickups and in particular to a new and useful sound pickup which operates on the principle of a magnetodynamic or electromagnetic transducer and in which a permanent magnet or a soft iron part, connected to a phonograph needle, is mounted for oscillation in front of or between parallel pole rods which are provided with coils and which are connected at one of their ends to a common pole plate.
2. DESCRIPTION OF THE PRIOR ART
The mechanical oscillations which are picked up by a needle of a sound pickup are converted into a corresponding audio frequency alternating voltage by a variation of the flux in a magnetic circuit. The lines of force of the magnetic flux penetrate coils provided on pole rods in the sound pickup. In magnetodynamic systems, the magnetic flux is produced by a permanent magnet which is mounted in front of or between the free ends of the pole rods and which is moved by the needle. In electromagnetic systems the permanent magnet producing the flux is firmly connected to the pole plate carrying the pole rods and the variations of the flux are produced by a soft iron armature which is mounted for oscillation in front of the free ends of the pole rods and moved by the phonograph needle. In these prior art constructions, which are of uniform design, an undesired effect is produced in that, because of the influence of the acting magnetic forces, the pole rods tend to oscillate during operation and such oscillation produces mechanical vibrations which lie in the acoustic range and unfavorably affect the transmission response of the pickup. In particular, peaks and troughs appear in the characteristic of the transducer. Even with a suitable dimensioning of the pole rods and coils it is difficult to reduce the vibrations.
Another known provision for reducing the tendency to oscillations is an embedding of the entire magnetic system in a plastic. Such constructions however have the disadvantage of great weight, which is undesirable with pickups.